Scientists and clinicians face a number of challenges in development of therapeutically active agents into forms suitable for delivery to a patient. For example, pharmaceutically useful biomolecules, including antibodies and antibody fragments, can now be prepared on a useful scale, due to advances in biotechnology, but the clinical usefulness of potentially therapeutic biomolecules is often hampered by their rapid proteolytic degradation, their instability upon manufacture, storage or administration, and/or their immunogenicity. Therapeutically potent molecules can be hampered by low aqueous solubility, paclitaxel (Taxol™) is one significant example.
Conjugation of active agents to water-soluble polymers has been found to reduce immunogenicity and antigenicity, as well as increasing half-life in circulation, as a result of decreased clearance and/or decreased enzymatic degradation in systemic circulation. The frequency of administration can thus be reduced, which is particularly beneficial in the large number of cases in which the agent is administered by injection. As a further benefit, active agents that are only marginally soluble in water often demonstrate a significant increase in water solubility when conjugated to a water soluble polymer.
Polyethylene glycol, due to its documented safety and its approval by the FDA for both topical and internal use, has been conjugated to a variety of active agents. Such a conjugated active agent is conventionally referred to as “PEGylated.” Commercially successful PEGylated active agents include PEGASYS® PEGylated interferon α-2a (Hoffmann-La Roche, Nutley, N.J.), PEG-INTRON® PEGylated interferon α-2b (Schering Corp., Kennilworth, N.J.), NEULASTA™ PEG-filgrastim (Amgen Inc., Thousand Oaks, Calif.) and SOMAVERT® pegvisomant, a PEGylated human growth hormone receptor antagonist (Pfizer, New York, N.Y.). Non-peptidic small molecules such as fluorouracil (Ouchi et al., Drug Des. Discov. 9(1):93-105, 1992) have also been prepared in PEGylated form.
In view of these promising features, there has been an increasing need for high purity functionalized derivatives of PEG and other water soluble polymers having medium to high molecular weight. However, the synthesis of such compounds is often complicated by the difficulty in removing polymeric impurities that accumulate during multi-step preparations. For example, end-capped (i.e., nominally monofunctional) polyethylene glycol starting material often contains significant amounts of PEG diol impurity, ranging from 0.5% to over 30% by weight. The diol impurity, and especially its reaction products when carried through a series of synthetic transformations, can be extremely difficult to analyze and remove. Higher molecular weight polymeric side-products, in particular, are generally quite difficult to remove and require time-consuming and expensive chromatographic techniques. Accordingly, there remains a need in the art for improved methods of preparing functionalized derivatives of water soluble polymers such as PEG.